Modern products have become increasing sophisticated, requiring complex electrical interconnections to provide power, control, and monitoring functionality to various assemblies and components within the product. For example, motor vehicles, such as passenger cars, often have scores of embedded computers to provide monitoring and control for operation, safety, convenience, information, entertainment, and emission control purposes. These processors need to be connected to the assembly that they monitor or control, and usually need to interconnect to some central display or control facility. Also, a vehicle is loaded with scores of electronic devices, such as a radio, electric seats, power windows, entertainment systems, navigation, emission and safety monitors, lighting, cameras, and many other devices currently used and even more importantly for future devices that will rely on higher speed data communication formats. All of these require power, and most need connection to buttons, switches, other devices, a computer, or operator display. In all, hundreds and hundreds of electrical and communication lines run throughout the vehicle. It is critical for operation, safety, emission control, and comfort that these lines provide for a robust and confident electrical and communication connection. Failure of the electrical or communication connections can lead to failure of the vehicle, unsafe conditions, customer dissatisfaction, and expensive and complex warranty repairs. In another example, wiring harnesses are also used in aircraft, spacecraft, marine and agricultural products and vehicles as well as machinery, appliances, instrumentation and electronic devices and systems.
To provide for ease of assembly and increased protection of the electrical lines, the lines are often routed around the vehicle in a multi-circuit wire bundle. This wire or cable assembly, often referred to as a harness, comprises wires and connectors that may be constructed using various types of machinery or built by hand. Then, at a later time in the manufacturing process, the harness is assembled into a final product. For example, FIG. 2 shows a relatively simple cable harness for a modern passenger vehicle, which will be discussed in detail later.
Manufacturing requirements for these cable assemblies may reference industry standards (such as SAE) or may have contractual specifications that must be verified. Many cable assembly manufacturers use a simple continuity test and in some cases a DC resistance measurement to determine that a cable assembly has been manufactured correctly. Continuity tests and DC resistance measurements are limited in detection capabilities to catastrophic faults such as opens and shorts but do not verify that industry or contract required construction standards have been met. Current verification methods often require destructive testing of the assembly to allow physical measurement and visual inspection. Destructive testing is conducted only on a sample basis meaning that large numbers of cable assemblies are shipped to customers with only a limited test that is not capable of detecting manufacturing defects such as improper crimp height, missing wire strands or insulation in the connector pin crimp, insulation cuts or chafes and broken wire strands.
Typical cable assembly testing consists of a continuity test between connectors that shows that the wire has an electrical connection to the correct pins on the connectors. This type of testing does not provide any measurement information that can be used to determine the conformance and quality of the assembly. Other measurement techniques could be used to better identify the electrical properties of the cable assembly to verify the quality of the materials and construction. One technique is to use Time Domain Reflectometry (TDR) to measure the impedance of the assembly along its length. Measurements of the previously listed cable defects have shown that a Time Domain Reflectometer (TDR) would require a few milli-ohms of impedance resolution and a length resolution of 0.25 inches or better for successful defect detection. TDRs with this type of performance typically cost $100,000 or more and are limited to testing one wire pair at a time. Large harness assemblies can have hundreds of wire pairs requiring hours of test time with a standard TDR, and therefore TDR has not been a practical option for harness manufacturers.
The substantial limitations in current cable testing procedures and devices leads to two different and very undesirable consequences. First, cable harnesses are shipped to assembly facilities with an existing undetected defect, and the defect is not discovered until sometime later in the assembly process. This causes unnecessary delay and disruption to fast-moving modern manufacturing processes, leading to down-time and increased manufacturing costs. For example, an assembly plant may assemble an entire vehicle, and then discover in final test that some electrical failure exists because of a faulty cable harness. Second, and even worse, the cable assembly might pass all the factory tests, but then one or more electrical lines fail soon after delivery to a customer. Still worse, such latent wiring defects are often intermittent, making it harder, more expensive, and more frustrating to identify the problem. This leads to potential safety issues, customer dissatisfaction, and a potentially expensive and time-consuming warranty repair. Since the cable harnesses are such a central part of a vehicle, having to replace one can often only be done with a major removal or disassembly of instrument panels, engine components, seats, carpeting or body panels. It has been estimated that almost 25% of warranty claims are related to these later discovered wiring defects, and these claims are individually relatively expensive.